The present invention relates generally to magnetoresistive sensors, and more particularly, to the self initialization of short magnetoresistive sensors into a single domain state.
It has long been recognized that instability in magnetoresistive sensors is produced by multiple domain states. In the past it has often been necessary to enhance stability by initializing the magnetoresistive sensor into a single domain state prior to operation by the application of an external longitudinal field of sufficient strength to saturate the sensor.
This conventional technique has numerous problems. The initialization only occurs during manufacture and not in the disc drive itself. Multiple domain states are produced during operation by large fields produced by the head during writing, or by fields from the disc itself. Stray fields that might occur during shipping or assembly also produce multiple domain state. These states often remain during normal operation of the head and cause instability. In addition, because the magnetoresistive sensor is shielded, it may be difficult to produce a strong enough field to entirely remove all of the domains from the sensor.
In coupled magnetic films, such as those present in dual element sensors, the most stable state is to have the magnetization associated with one element antiparallel to the other element. However, an external field can only produce parallel alignment of the magnetizations. Under the influence of the write field, or stray fields, this state may relax to the antiparallel state. During this relaxation, or afterwards, a multiple domain state may be produced.
Typical conventional structures are described in "Magnetic Self-Bias in the Barber Pole Structure," by J. S. Y. Feng et al., in IEEE Transactions on Magnetics, Vol. MAG-13, No. 5, September 1977, "The Design of Magnetoresistive Multitrack READ Heads for Magnetic Tapes," by Werner Metzdorf et al., in IEEE Transactions on Magnetics, Vol. MAG-18, No. 2, March 1982, "A specific model for domain-wall nucleation in thin-film Permalloy microelements," by Neil Smith, in Journal of Applied Physics, Vol 83, No. 8, April 15, 1988, and "Study of domain formation in small permalloy magnetoresistive elements," by C. Tsang et al. in Journal of Applied Physics, Vol 52, No. 38, March 1982.
One conventional technique enables a longitudinal field to be applied to the magnetoresistive element directly by the sense/bias current (see the Feng et al. article cited above). In test samples substantiating that invention it was found that the stabilizing fields produced by the sense/bias current in conjunction with a uniform saturating transverse field are capable of reorienting the magnetization in a single magnetoresistive element producing a stable magnetoresistive response. However, the structure that applies the longitudinal field is not able to completely guarantee a single domain state in the magnetoresistive element for a number of reasons.
The longitudinal fields favor a parallel alignment of the magnetization so that at the ends of the magnetoresistive element the lack of closure will introduce a multiple domain state. To minimize this effect the magnetoresistive elements are made relatively long compared with the active area so that the end domains are far away from the active area. The possibility exists that, under the influence of stray fields, the multiple domains could come sufficiently close to the active area to affect the stability of the device.
The region over which the stabilizing longitudinal field is strong is relatively close to the active area. Areas away from the active area are only stabilized by the exchange coupling of the magnetization in the element. If these outside regions were affected by stray fields from adjacent tracks, or in anyway become multidomain, then the magnetization in the active area could be affected by the exchange coupling of the magnetization in the element.